Method of manufacturing coated elements for electron tubes



Feb. 27, 1951 E. A. COOMES ETAL 2,543,439

NG COATED METHOD ANUFACTURI ELEME R ELECTRON TUBES Filed May 2, 1945 FIG. I

F IG. 2 '0 7% PRESS HEAD -30 I PRESSING CYLINDER 5 i I3 4; g k Mil/gi l? N65 \EJECTING COLLAR A FIG.4 4V, m CATHODE\ 1 LEEVE WITH E DSPUN I F0 OUNTING CHAN NEL SECTION CORRUGATED STRIP THREE LEG STRIP SUPPORT BSE F|G.5 4o FLARED END OF NI SLEEVE 45 N2;

SINTERED SECTION ON SLEEVE I INVENTORS 50 RD A.COOMES PE R w. FORSBERGH JR A T TORNEY Patented Feb. 27, 1951 UNITED STATES PATENT OFFICE METHOD UFACTURING COATED ELEMENTS FOR ELECTRON TUBES Application May 2, 1945, Serial No. 591,586

7 Claims.

Our invention relates in general to methods for producing cathodes for thermionic tubes and more particularly to a method for producing so called sintered cathodes.

In thermionic tubes it is desirable, particularly for some applications thereof, to provide a cathode having a high thermal emissivity and also, in addition. a good secondary emission characteristic. Accordingly, it is one of the objects of our invention to provide a cathode of this nature.

In addition there are distinct advantages to having a cathode having a uniformly machined hard surf-ace since it reduces the likelihood of sparking within the tube. Accordingly, it is another of the objects of our invention to provide such a cathode.

Also it is advantageous if the emission material may be made comparatively thick since this increases the length of life of the tube and therefore it is another of the objects of our invention to provide such a cathode,

Our invention will best be understood by reference to the hereinafter disclosed method and to the drawings, in which:

Fig. 1 shows a die for forming short cylindrical sections of cathodic material;

Fig. 2 shows an ejecting collar used with the press of Fig. 1;

Fig. 3 shows a tray which may be used in sintering a number of cathodes simultaneously;

Fig. 4 shows a side view of the arrangement in accordance with Fig. 3 with a representative cathode shown in place; and

Fig. 5 shows a mandrel arrangement with a cathode fastened thereto.

In the preparation of the cathode a mixture of nickel powder and barium carbonate and strontium carbonate is used with a small trace of magnesium included therein as an activator. In actual practice, the nickel powder which has been of 200 mesh or finer has comprised 50% to 60% by weight of the total powder of the material which is to be sintered, and the powder comprises magnesium from .01% to .03% by weight and which may be an activator. The barium carbonate and the strontium carbonate are mixed in a ratio which varies between 2:1 and 1:1 with respect to each other and make up substantially the remainder by weight of the powder. Stearic acid crystals (approximately 1% by weight) are dissolved in cold acetone and the carbonates of the barium and strontium and the magnesium are added and the mixture is ballmilled for 10 to 20 hours. The nickel is then added and milled for an hour. The acetone is evaporated in a dish by blowing warm air and continuously stirring the mixture with a clean nickel spatula. It will be appreciated that the proportions hereinbefore discussed are not limiting by nature but are those which have been used practically and which have formed a preferred type of cathode structure. Also that the times of milling may vary. Weighed quantities of the powder that has been formed by the mixture of barium and strontium carbonates and of nickel powder and magnesium are pressed into small cylindrical forms by placing predetermined quantities of the powder in a die, such for instance as shown hereinafter in Fig. 1. The powder is subjected to a pressure of between 10 to 15 tons per square inch for approximately 1 minutes, and the pressure is released slowly.

The small cylinders so formed, and which in practice have been 4 to 7 millimeters long with a concentric hole therein, may be stacked or beaded onto a loosely-fitted nickel sleeve and baked in dry hydrogen for a period of time wherein the temperature of the oven is raised slowly for approximately of an hour to 1 hour to 1200 C. and held for a period of from 15 to 25 minutes during which sintering of the mixture of the small cylinders takes place; During the raising of the temperature of the oven, it is better to hold for a period of from 10 to 15 minutes when the temperature reaches 1000" C. in order to free the carbon dioxide from the mixture as completely as possible before appreciable sintering takes place. The result is a series of very hard, unwarped, grayish beads separated from each other by a space around millimeter and which cannot be twisted off of the nickel sleeve by hand. Sectional cuts show the nickel sleeve and the sintered coating thereon to be essentially one continuous piece of metal for the whole length of the pellet.

The sleeve may be placed on a mandrel, shown hereinafter in Fig. 5, and turned down to the desired diameter by fast feed cuts from .003 inch to .005 inch in depth. finished by two slow-feed shallow cuts and a final deburring with a wire brush or polishing with a clean fine file to re- .duce sparking in the cathode. "The cathode then 66 ment of a die and press which may be used in forming the small cylinders of the milled powdered miitture which later are to be mounted on a nickel sleeve and sintered thereon. In this figure, a press head I has positioned adjacent thereto a solid pressing cylinder I l. Adjacent the pressing cylinder is a spring-mounted frame I2, which contains a die section comprising a substantially cylindrical opening I3, having a solid section l4 passing axially therethrough. Positioned adjacent the openings l3 is a fixed member l6 which acts to help eject the formed, pressed cylinders of powder after pressure has been applied thereto by the press head. The frame I2 is positioned to move along a guide member having two or more arms 20, the latter having springs positioned thereabout which are adapted to engage the frame [2 when pressure is applied by the press head. These guides or circuits are fastened to a base member 2 l The cylincer formed by this method has been, in practice, from 4 to 7 millimeters long with a concentric axial hole therein.

Referring to Fig. 2, there is shown an ejecting collar for use with the press of Fig. 1. After the cylinders of powder have been pressed in the openings IS, the solid pressing cylinder ll may be replaced by the ejecting collar 30, the latter being merely a cylindrical piece of steel with comparatively thick side walls. It will be obvious that when the press head forces the ejecting collar 30 against frame member l2, the frame will be forced down against the springs 20 and the fixed member l6 and force the formed cylinder up out of apertures l3 and into the collar.

Referring to Fig. 3, there is shown a pan arrangement for firing and sintering the material placed on the nickel sleeve holders. This member is of the general shape shown in the drawing and comprises a corrugated nickel sheet. with a channel section against which the members may rest while undergoing firing.

Referring to Fig. 4, there is shown a side view of the arrangement according to Fig. 3 with the addition of a representation of a nickel cathode sleeve with the material to be sintered thereon placed about the sleeve. flared nickel strip has been placed over the top of the row of sleeves which are placed in the corrugations in the nickel corrugated sheet of the baking pan to prevent any solid matter from falling off the cathodes. The tray may be slid into a cold oven fed with dry hydrogen and treated in the manner hereinbefore set out in this specification.

Referring to Fig. 5, there is shown a mandrel 40 having an end section against which is placed the flared end of the nickel mounting sleeve of the cathode. This fits about an elongated cylindrical arm 4|, which is threaded at one end to accommodate a nut 42. The nut 42 then may force, by means of members 43 and 44, the flared end of the cathode up in intimate contact with the enlarged section of the mandrel where it is ,held

tightly in position. The mandrel then may be mounted on a lathe arrangement for cutting or turning down to the right diameter or desired diameter the cathode fastened thereon.

Activation of the cathode so formed may be accomplished by heating for several hours to a temperature from 1000 C. to 1050" C. and then at 950 C. for a time up to approximately 30 minutes. Further glowing at 950 C. in sealed-off glass tubes is generally beneficial. In practice, the D.-C. emission at 950 C. has been around 200 maJcm. and the microsecond pulsed emission of In actual practice, a

at least 10 amp/cm. and currents as high as 25 amp/cm. have been obtained.

It will be appreciated that what has been described hereinbefore is in the nature of a preferred embodiment and that there will be deviations therefrom which fall fairly within the spirit and scope of the invention, and accordingly we claim all such deviations that fall within the provisions of the hereinafter appended claims.

We claim:

1. Method of forming a cathode element which comprises the steps of mixing nickel powder with powdered compounds of barium and strontium, binding the resultant mixture with a binder, molding the resultant bound mixture under pressure into elements of desired" shape and size, loosely mounting a plurality of the molded elements on a cathode supporting surface, and heating the combined supporting surface and molded elements to form a sintered deposit in close adherence to the supporting surface.

2. Method in accordance with claim 1 wherein the binder is stearic acid.

3. Method in accordance with claim 1 wherein there is included the additional step of milling the mixture after the combination therewith of a binder and before the step of molding the mixture into the elements of a desired size and shape.

4. The method of manufacturing an electron tube cathode comprising the steps of combining in powdered form the substances of which the outer surface of the cathode is to be composed, binding the mixture with a binder, molding the bound mixture under pressure into a plurality of hollow elements, threading said elements over a loosely-fitting supporting member, heating the elements and their supporting member to cause said elements to form a sintered coating on said member, and machining said sintered coating to the desired size and shape.

5. The method of manufacturing an electron emitting cathode comprising the steps of combining in powdered form the substances of which the outer surface of the cathode is to be composed, binding the mixture with a binder, molding the bound mixture into a plurality of cylindrical elements, each with a concentric hole therein, threading said elements over a looselyfitting, hollow supporting sleeve, heating the ele-- ments and their supporting sleeve in an a phere of dry hydrogen to form a sintere k ing on said sleeve, fastening saidfslee'v 'onia mandrel, and machining said coatir'ig ,to the desired shape and size.

6. The method of manufacturingia' cathode for an electron tube comprising the steps of mixing substantially equal parts of barium carbonate and strontium carbonate with a trace of magnesium; adding said mixture to cold acetone in which stearic acid crystals have been dissolved; ball-milling the resulting mixture for ten to twenty hours; adding nickel powder to the milled mixture so that thefinal proportion by weight of nickel to the mixture of nickel, barium carbonate and strontium carbonate is 50%-60% nickel; ball-milling the mixture for an additional hour; evaporating the acetone from the mixture; molding the residue under said sleeve in an atmosphere of dry hydrogento 10m a. sintered coating oi said elements on said sleeve.

7. The method of claim 6 including the further steps 01' rigidly mounting said nickel sleeve on a mandrel and machining said sintered coatin: to the desired shape and dimensions.

EDWARD A. COOMES. PETER W. FORSBERGH, JR.

REFERENCES CITED The following references are of record in the tile or this patent:

UNITED STATES PATENTS Number Name Date Reeve July 2, 1929 Gero Oct. 15, 1929 Lowry Aug. 21, 1934 Robinson Apr. 6, 1937 Espe June 21, 1938 Felsner Jan. 2. 1942 Bondley Mar. 10, 1942 

1. METHOD OF FORMING A CATHODE ELEMENT WHICH COMPRISES THE STEPS OF MIXING NICKEL POWDER WITH POWDERED COMPOUNDS OF BARIUM AND STRONTIUM, BINDING THE RESULTANT MIXTURE WITH A BINDER, MOLDING THE RESULTANT BOUND MIXTURE UNDER PRESSURE INTO ELEMENTS OF DESIRED SHAPE AND SIZE, LOOSELY MOUNTING A PLURALITY OF THE MOLDED ELEMENTS ON A CATHODE SUPPORTING SURFACE, AND HEATING THE COMBINED SUPPORTING SURFACE AND MOLDED ELEMENTS TO FORM A SINTERED DEPOSIT IN CLOSE ADHERENCE TO THE SUPPORTING SURFACE. 